One of the major problems facing today""s society and future generations is the production of air pollution by a variety of combustion systems, such as boilers, furnaces, engines, incinerators and other combustion sources. Air pollutants produced by combustion include particulate emissions, such as fine particles of fly ash from pulverized coal firing, and gas-phase (non-particulate) species, such as oxides of sulfur (SOx, principally SO2 and SO3), carbon monoxide, volatile hydrocarbons, volatile metals (i.e., mercuryxe2x80x94Hg), and oxides of nitrogen (mainly NO and NO2). Both NO and NO2 are commonly referred to as xe2x80x9cNOxxe2x80x9d because they interconvert, the NO initially formed at higher temperature being readily converted to NO2 at lower temperatures. The nitrogen oxides are the subject of growing concern because of their toxicity and their role as precursors in acid rain and photochemical smog processes.
One other major problem facing society is the ever expanding consumption of and dependence on energy, including specifically fossil fuels. One area of great promise for better, more efficient and environmentally conscious energy usage is in the utilization of waste fuels for energy production. Large quantities of agricultural and other biomass resources are available throughout the world. Biomass is a renewable source of energy, but a lot of this material is being land filled, burned in the open fields, or plowed under and, thus, are not utilized as an energy feedstock. Utilization of biomass for energy production eliminates costs for its disposal, provides a renewable energy resource and decreases CO2 emissions. Currently, due to slagging and fouling of boilers"" heat transfer surfaces, biomass boilers cannot use a variety of bio-feedstocks with high alkali content.
Accordingly, two key needs are 1) decreasing NOx emissions from combustion sources and 2) increasing utilization of low-grade waste fuels for energy production.
There are several commercial technologies that are available to control NOx emissions from stationary combustion sources. Combustion modifications such as Low NOx Burners (LNB) and overfire air (OFA) injection provide only modest NOx control, on the level of 30-50%. However, their capital costs are low and, since no reagents are required, their operating costs are near zero. For deeper NOx control, Selective Catalytic Reduction (SCR), reburning, Advanced Reburning (AR) or Selective Non-Catalytic Reduction (SNCR) can be added to LNB and OFA, or they can be installed as stand alone systems.
Currently, SCR is the commercial technology with the highest NOx control efficiency. With SCR, NOx is reduced by reactions with N-agents (ammonia, urea, etc.) on the surface of a catalyst. The SCR systems are typically positioned at a temperature of about 700xc2x0 F. SCR can relatively easily achieve 80% NOx reduction. However, SCR is far from an ideal solution for NOx control. There are several important considerations, including cost. SCR requires a catalyst in the exhaust stream. Catalysts and related installation and system modifications are expensive. In general, SCR catalyst life is limited. Catalyst deactivation, due to a number of mechanisms, typically limits catalyst life to about four years for coal-fired applications. In addition, catalysts are toxic and pose disposal problems.
Reburning is a method for controlling nitrogen oxides that involves combustion of a fuel in two stages. FIG. 1 may be referred to in this discussion concerning reburning techniques. As shown in the reburning system 100 of FIG. 1, in the main combustion zone 102 80-90% of the fuel is burned with normal amount of air (about 10-15% excess). This corresponds to an Air/Fuel Stoichiometric Ratio (SR) about 1.10-1.15. The combustion process forms a definite amount of NOx. Then, in the second stage, the rest of the fuel (reburning fuel) is added at temperatures of about 2300-3000xc2x0 F. into the secondary combustion zone 104, called the reburning zone, to generate a fuel-rich environment. Test results indicate that in a specific range of conditions (equivalence ratio in the reburning zone, temperature and residence time in the reburning zone) the NOx and N2O concentrations can typically be reduced by 50-60%. In the third stage 106 the OFA is injected at a lower temperature to complete combustion. Typically the OFA is injected at 1800xc2x0 F.-2800xc2x0 F. to achieve essentially complete combustion.
The flow diagram section, b, of FIG. 1 illustrates the main reactions in the reburning zone process. Adding the reburning fuel leads to its rapid oxidation by the excess oxygen to form CO and hydrogen. The reburning fuel provides a fuel-rich mixture with certain concentrations of carbon containing radicals 108, e.g., CH3, CH2, CH, C, and HCCO, which can react with NO. The carbon containing radicals (CHi) formed in the reburning zone are capable of reducing NO concentrations by converting it to various intermediate species with Cxe2x80x94N bonds, 110. These species are reduced in reactions with different radicals into NHi species 112, e.g., NH2, NH, and N, which react with NO to form N2 114. N2O is reduced mainly via reaction with H atoms: N2O+Hxe2x86x92N2+OH. The OFA added on the last stage of the process oxidizes existing CO, H2, HCN, and NH3.
Typically, reburning fuel is injected at flue gas temperatures of 2300-3000xc2x0 F. The efficiency of NOx reduction in reburning increases with an increase in injection temperature. This is because at higher temperatures oxidation of the reburning fuel occurs faster, resulting in higher concentrations of carbon containing radicals involved in NOx reduction. Efficiency of NOx reduction also increases with an increase in the amount of the reburning fuel at reburning fuel heat inputs of up to 20-25%. Larger amounts of reburning fuel practically do not increase and sometimes even slightly decrease the efficiency of NOx reduction.
Conventional reburning typically requires 15% to 20% reburning fuel heat input to achieve 40%-60% NOx reduction. In so-called Fuel-Lean Reburning (FLR) the amount of the reburning fuel is controlled to maintain an overall fuel-lean stoichiometry in the upper furnace. Therefore, no additional OFA is required for completing burnout. FLR has shown the potential to achieve about 25-35% reduction in NOx emissions using 7-8% natural gas heat input or less.
Greater levels of NOx control can be achieved using Advanced Reburning (AR) techniques. AR is a synergistic combination of basic reburning and N-agent (ammonia or urea) injection. Initial AR studies focused on N-agent injection into the burnout zone (AR-Lean). It was found that AR-Lean incorporates the chain branching reaction of CO oxidation which promotes the reaction between NO and ammonia. When CO reacts with oxygen, it initiates many free radicals. Experiments and modeling studies have demonstrated that the de-NOx temperature window can be substantially broadened and NO removal efficiency increased, if both CO and the O2 concentrations are controlled to fairly low values (CO at the order of 1000 ppm and O2 at less than 0.5 percent). At the point of air addition, CO and O2 are both at low values because of the close approach to SR=1.0, yielding about 85% NO reduction.
Injection of small amounts of alkali promoter species, such as sodium carbonate, along with ammonia into the reburning zone (AR-Rich) can further improve upon the AR process. These AR improvements are capable of achieving greater than 90% NOx control.
Waste fuels can be very effective for reburning. Tests with several feedstocks (yard waste, furniture manufacturing sawdust, walnut shells, willow wood, waste coal fines and others) demonstrate that advanced waste reburning technologies can achieve higher NOx reduction even than that achieved with natural gas. Efficiency of NOx reduction for most waste fuels increase with an increase in the amount of the reburning fuel.
In one technique, biomass pyrolysis gas serves as the reburning fuel. Pyrolysis-based units produce gas, char and tar. Using a reburning technique, pyrolysis products are injected in a combustor as reburning fuel at different temperatures of pyrolysis and various air/fuel stoichiometric ratios in the combustor""s reburning zone. Maximum NOx control performance of 87% has been achieved with biomass gas combined with the tar formed at pyrolysis temperature of 1650xc2x0 F. At a stoichiometric ratio of 0.8, biomass gas has exceeded the performance of natural gas, which was about 75%.
A number of efforts have been made to utilize waste fuels for energy production. One driving force to make fuel-flexible power technologies less costly than conventional fuel power technologies is the low or negative cost associated with opportunity fuels. Another reason is the societal goals, energy conservation, environmental conservancy and care, and others. Large quantities of opportunity fuels including urban wood waste, agricultural residues, forest waste, municipal solid waste, and sewage sludge are land filled and, accordingly, their beneficial uses are unrealized. These feedstocks are low-grade waste fuels.
There are several technologies that are available to produce energy from waste fuels. Direct combustion involves the burning of fuel with excess air, producing hot flue gases that are used to produce steam in the heat exchange sections of boilers. The steam is used to produce electricity in steam turbine generators.
Direct combustion of waste fuels has proven inefficient because of poor combustion characteristics generally associated with waste fuels. When compared to fossil fuels, waste fuels have a heterogeneous composition, sometimes high ash and/or moisture content, low heating value, substantial chlorine content, and trace heavy metal content. Because of that, existing biomass boilers are limited in efficiency and suffer undesirable consequences of fuel ash fouling. Combustion of these fuels requires expensive solids handling equipment, corrosion protection, high excess air, scrubbers, filters, and other air pollution control systems.
Co-firing refers to the practice of introducing biomass or waste fuels in the main combustion zone of fossil fuel fired boilers as a supplementary energy source. Co-firing has been evaluated for a variety of boiler technologies including pulverized coal combustors, fluidized bed units, and stokers. Because waste fuel comprises only a fraction of fossil fuel, negative impact of waste fuel on boiler performance is reduced in co-firing.
As an alternative for direct waste combustion, gasification can be applied to a variety of waste products, providing a cleaner gaseous fuel. The gasification process takes solid waste products and improves its combustion characteristics, handleability, and simultaneously may reduce pollutant emissions. Gasifiers can frequently handle high fouling fuels without excessive slagging/fouling due to the lower temperatures at which they can operate in comparison with direct combustion units. Waste fuel gasification generally involves heating fuel in an oxygen-starved environment to produce a medium or low calorific gas. This xe2x80x9cbiogasxe2x80x9d is then used as fuel in a combined cycle power generation plant that includes a gas turbine topping cycle and a steam turbine bottoming cycle, or can be used for co-firing in coal and biomass fired boilers.
The present invention is related to processes for removing emissions of nitrogen oxides in combustion systems. More specifically, the present invention provides methods for decreasing nitrogen oxides emissions from stationary combustion sources and for utilizing low-grade biomass and other waste fuels without slagging and fouling problems.
The present invention represents an improvement over prior techniques in that it presents methods and systems that effectively and efficiently reduce NOx while utilizing gasified fuels, including biomass and low-grade waste fuels. In general, the present invention unconventionally achieves these improvements by gasifying solid fuels and injecting produced gas into a reburning zone of a boiler at relatively low temperatures and in relatively small amounts. If the gas is fed into a reburning zone of a boiler, the gas cleaning requirement is eliminated or substantially reduced, as tars are burned in the flame and alkali species may be present at much lower levels than is the case with direct combustion applications.
Importantly, there are key differences between the present invention and prior techniques, including, for instance: 1) conditions in a gasifier are such that gasification products with optimum concentrations of nitrogen (N)- and alkali (Na and K)-containing species are produced; 2) specific flue gas temperature of the boiler at which the gaseous products are injected is selected, and 3) reaction time in the post-combustion or reburning zone for effective interaction of the N- and alkali-containing species in the gasification products with NOx in flue gas is provided.
In addition, the present invention improves over prior techniques by realizing a very high efficiency of NOx removal by gasification products. Generally, propane and natural gas have been thought to be the most effective reburning fuels with coal being slightly less effective. Also, the efficiency of syngas, with CO and H2 being its major components, is much less than that of propane and natural gas. Implementation of the present invention yields the surprising result that efficiency of syngas, such as from waste gasification, under optimized conditions can be higher than that of propane and natural gas, as shown in the examples set forth in the detailed description hereinbelow. Moreover, the invention achieves efficiencies of 70% NOx reduction and higher at 6%-8% reburning fuel heat inputs. This result is quite remarkable and unexpected since in Fuel-Lean Reburning only 25-35% reduction in NOx emissions is obtained using 7-8% natural gas heat input.
Particular embodiments of the invention provides a method of decreasing emissions of nitrogen oxides (NOx) in combustion systems in combination with utilization of at least one low grade solid fuel. The inventive method comprising the steps of: causing the combustion of a main fuel in a combustion system, thereby resulting in the generation of a combustion flue gas in a post combustion zone, the combustion flue gas comprising nitrogen oxides; gasifying at least one solid fuel in a gasifier causing the generation of a gaseous product containing solid particles, the gaseous product comprising one or more of the group consisting of carbon monoxide, hydrogen, hydrocarbons, water, carbon dioxide, ammonia and other reduced N-containing species, and small amounts of alkali-containing compounds; and injecting the gaseous product into the post combustion zone of the combustion system to create a reaction zone in which nitrogen oxides are reduced to molecular nitrogen by introducing the gaseous product into the post combustion zone at a temperature designed to promote reaction of NO with one or more of the group comprising syngas components, CO, H2, hydrocarbons, ammonia, and N- and alkali-containing compounds.
In another embodiment, the invention provides a combustion system for causing the combustion of fuel, the combustion of fuel resulting in the generation of post-combustion flue gas, including NOx, the combustion system comprising: a primary combustion zone in which the combustion of a main fuel occurs, the combustion of the main fuel generating flue gas, which exit the combustion zone; a post-combustion zone for receiving the flue gas; and a gasifier receiving biomass or waste fuel and producing a gaseous product at least in part therefrom and delivering the gaseous product into the post-combustion zone for reacting with the flue gas to reduce NOx emissions, the gaseous product being introduced into the post combustion zone at a temperature designed to promote reaction of NO with one or more of the group comprising syngas components, CO, H2, hydrocarbons, ammonia, and N- and alkali-containing compounds.
It is therefore an object of the present invention to provide methods for eliminating or at least dramatically reducing nitrogen oxides from combustion flue gas before they are emitted to the atmosphere.
It is an object of the invention to introduce gaseous products into a post combustion zone at such a temperature so as to promote the reaction of ammonia and alkali-containing compounds with NO contained in flue gas to reduce NOx emissions.
It is another object of the present invention to decrease the concentration of nitrogen oxides formed in combustion by injection gasification products of different fuels into a post combustion or reburning zone of a combustion system.
It is another object of the present invention to utilize biomass and low-grade waste fuels with high fuel-N and alkali content for production of syngas.
Additional objects and advantages of the present invention will be apparent to those skilled in the art upon reading the description and claims and examining the figures, or may be learned by the practice of the invention.